The present invention relates to a heater for liquid containers such as storage tanks and hot tubs and more particularly to such a heater adapted to permit direct heat exchange between the contained liquid and a thermally conductive plate which is heated by a heating element.
Hot tubs are becoming increasingly popular among people wishing to enjoy either the therapeutic or relaxing effect of hot water soaking. The tubs may be formed in a variety of sizes or shapes and may be used either indoors or outdoors. Most commonly, the tubs are used out-of-doors and are constructed from a suitable wood such as redwood, yellow cedar or teak in order to resist deterioration from the hot water and also from normal weathering. The tubs may also be formed from other materials such as plastic, for example. The tubs may be either rectangular or round. A typical size is four to seven feet in diameter with a depth of from three to four feet. A tub in this size range would have a typical capacity of about 500 to 800 gallons.
In any event, it is desirable to be able to maintain the temperature of the water at an elevated temperature, for example, within the general range of from 100.degree. to 120.degree. F. (approximately 38.degree. to 49.degree. C.).
In order to heat water within hot tubs, it has been common practice to employ an external conduit including a pump and water heater capable of drawing water from the tub, heating the water and returning it to the tub at a rate necessary to mantain a desired temperature within the tub. This method of heating is of course similar to that employed for swimming pools. In fact, heaters and pumps developed for use with large capacity swimming pools have been commonly employed for hot tubs. Since a conventional hot tub may have a capacity of only about five to ten percent that of a swimming pool, it is obvious that such pumps, heaters and other components are of an excessive size and capacity when used with hot tubs.
The cost of accessory items such as heaters, pumps and filters therefore comprises a substantial portion of the cost for installing a hot tub. In addition, such systems tend to be relatively inefficient because of the need to circulate water between the tub and the heater. Even further, the necessary use of a pump for circulating water between the tub and the heater produces substantial undesirable noise.
Accordingly, there has been found to remain a need for a simple, efficient and economical heater for use with hot tubs and other liquid containers or storage tanks, particularly relatively large volume tanks of a type used for example in solar heating systems.